To study cross-fertilization in the freshwater pulmonate vector snail of human urinary schistosomiasis, Bulinus (Physopsis) globosus. The genetic marker albinism-pigmentation will be used to determine the objectives. The objectives include determination of frequency of copulation possible; rate of sperm production following copulation as males; age at which snails first act as male and produce viable sperm; age at which snails can first use foreign sperm as females; longevity of use of foreign sperm from single and multiple copulations; sperm competition between two males to fertilize the eggs of the same female; copulation as males by castrated snails and effect of a copulation with a castrated snail on egg production in a virgin, self-fertilizing snail; and the effects of desiccation, starvation and temperature changes on the survival of foreign sperm.